Fill out some skipped tests
Generate correct property accessor names in test stubs
Fix property accessor names in test descriptions
Clean up temporary variable
Fix test describe block label
Simplify IRepository tests with a let block
Explicitely guard agains looking up GType 0

Since the info types sometimes return 0 as a GType, make sure
IRepository#find_by_gtype raises an error before potentially causing a
coredump later.
Make #get_property work with an empty GList derivative
Make #set_property work with a GList derivative
Make #get_property work with a non-empty GList derivative